A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The patterning device and substrate are normally supported by a support structure, which is connected to a positioning device to accurately position the patterning device and substrate respectively in accordance with certain parameters. The position of these positioning devices are determined by a displacement sensor (e.g. an interferometric device, linear encoder or capacitive sensor) and at least one reference sensor which determines the “null” of the positioning devices. This “null” is preferably a six degree of freedom reference point for the positioning device with respect to a certain frame structure. An important parameter is the stability of the reference point over time and temperature. The requirements of this drift over time and temperature get more and more stringent. Current reference sensors may not be able to meet these and expected future requirements.
Commonly used reference sensors are so-called position sensitive devices (abbreviated as PSD). A PSD is an optical sensor that can measure the position of a light spot in one or two dimensions on a sensor surface. A PSD at least includes a diode structure, a resistive layer and electrode contacts. An example of a PSD is known from U.S. Pat. No. 4,887,140 the contents of which is herein incorporated by reference in its entirety. FIG. 1 of this document shows a one-dimensional PSD, whereas FIGS. 2-5 show different types of two-dimensional PSD's. The diode structure of a PSD is reverse biased. Incident radiation on the sensor surface between the electrode contacts creates a photocurrent, and the resulting currents through the electrode contacts are a measure of the position of the centre of gravity of the incident radiation. However, in a PSD, the reference point of the sensor is drifting over time and temperature. These drifts can be caused by (amongst others) local heating of the radiation spot and the PSD's reverse bias which may causes in-homogeneities in the resistive layer of the PSD.